Just Zerg movement.
Zerg (ground) units move about 30% faster on creep (notable exceptions being: Queen, Hydralisk and Drones).
Additionally Zerg can build on creep and non-Zerg structures cannot be built on creep. You can actually use an Overlord to spread creep at a number of expansions to slow down your opponent's expansion rate. Zerg structures off creep will slowly lose life and eventually die.
The following units/buildings can produce creep:
- Creep Tumor (built from the Queen and other Creep Tumors)
- Overlord (Overseers Cannot, additionally this is temporary)
- Nydus Worm (the Worm, not the Network)
Creep will evaporate if left without a production source near by. Its important to note that creep evaporates from the edge inward. As a result, if you remove creep production in an area that is not covered by, but is surrounded by, creep production, the creep will not evaporate.
Since people asked the following units are affected by creep in different ways:
- Queen: 170%
- Hydralisk: 50%
- Drone: 0%